In the field of electronics in general and that of semiconductor components for detecting infrared radiation in particular, it is necessary to systematically check the performance of components before they are fitted in a more complex structure, especially in a detector. These components are used in infrared detectors which classically comprise unitary hybrid components consisting of detection circuits mounted on a silicon readout circuit and their nominal operating temperature is classically 70 to 200 K.
Because they are “quantic” detectors, the operating temperature dictated by the physical principle which they exploit is that of liquid nitrogen and it is therefore necessary to check the actual performance of components at this temperature of use.
The use of test equipment which is advantageously automated and currently referred to in the field in question by the term “cryoprober” is known. Such equipment, the general principle of which is described in relation to FIG. 1, classically comprises a measuring head (5) equipped with probes (6) which come into contact with the electrical interfaces of the semiconductor component and a motor-driven staging fixture (3) which is controlled by a viewing system and accommodates the component under test, this entire assembly being isolated in a chamber (1) which is generally a high-vacuum chamber which is cooled to a very low temperature by a flow of cryogenic fluid (4) which is typically liquid nitrogen.
Chamber (1) also comprises a viewing port (2) on its upper surface which is capable of allowing illumination of the component under test or enables the latter to detect a scene outside the chamber.
The component which is to be characterized is put in place and held in contact with staging fixture (3) by means of foil made of oxidizable steel, said staging fixture being motor driven so as to allow the electrical interfaces of the component to come into contact with the tip of the probes of the measuring head. The measuring head ensures activation and readout of information for each of the components by establishing contact with them sequentially.
As a result of this structure, the components to be characterized are therefore cooled only by thermal conduction as they come into contact with the cooled staging fixture and by means of a mechanical retention system.
Because of the expected performance in their eventual applications, the components only have very small inactive surface areas available to ensure such mechanical retention because the bulk of their central zone is optically active and their peripheral surround is mainly reserved for electrical wiring and interfacing. FIGS. 2 and 3 schematically show a top view and a perspective view of such a component respectively. Component (10) therefore comprises an optically active central zone (12) and an optically inactive peripheral zone (11). The electrical interfaces of the components are also shown and denoted by (13).
This being so, only components which have sufficiently large inactive zones which are much bigger than the clearance allowed when slicing silicon wafers can be tested in a high-vacuum cryoprober.
Ultimately, the objective is to achieve the following result:                make it possible to load batches of hybrid components into such a cryoprober;        ensure extensive compatibility with several component geometries;        preserve the physical and functional integrity of components and, especially, not affect the active zone or inactive zone as a result of mechanical retention;        be able to ensure nominal cooling of components without having to resort to adhesives or greases;        enable electro-optical characterization at temperatures controlled to within ±0.5 K.        
The electro-optical performance data resulting from characterization operations are essentially:                sensitivity to illumination;        signal-to-noise ratio;        defects, i.e. the number of pixels in the active zone of the component which cannot be used.        
Essentially, two types of such “cryoprobers” used to ensure characterization at very low temperature are currently known.
Firstly so-called “overpressured” cryoprobers: In such a system, the test chamber is pressurized by a scavenging gas which does not condense at the characterization temperature, said gas generally consisting of dry nitrogen.
The components to be characterized are held on the staging fixture by subjecting their rear surface to negative pressure. Thermal transfer is ensured by the residual gas layer between the component and the staging fixture as well as by convection of cold gas between the staging fixture and the cooled measuring head.
Although such “overpressure” cryoprobers offer good component cooling efficiency and also ensure that components are secured without any mechanical contact with their front surface, thus preserving their physical integrity, their use does have one drawback: high consumption of cryogenic liquid. Not only that, cooling by convection imposes limitations in the case of certain electro-optical characterization processes, especially noise measurements which are polluted by variations in parameters which are sensitive to thermal fluctuations.
So-called “high-vacuum” cryoprobers are also known. In this setup, the test chamber is subjected to a high vacuum. The components are secured on the cooled staging fixture with the aid of a grid fitted with stainless steel foil. Thermal contact between the components and the cooled staging fixture is ensured by mechanical pressure applied to the front surface by the foil, this force being exerted on the inactive zones of the circuits.
These high-vacuum cryoprobers have the advantage of being able to provide nominal characterization conditions which are equivalent to the envisaged ultimate application thanks to the absence of internal convection and they also consume little cryogenic liquid.
On the other hand, one comes up against design constraints which are necessary in order to guarantee mechanical retention and thermal contact between the component and the staging fixture; these constraints are generally incompatible with the miniaturization of components which is a constant objective for those skilled in the art.
The object of the invention is precisely to optimize retention of hybrid components on the cooled staging fixture without affecting the physical integrity of said component and without impacting the efficiency of actual cooling itself.